The invention relates to a method to modulate plant growth or development.
Apart from short-term environmental factors, such as the daily rhythm of night and day and the immediate availability of water and nutrients, long-term environmental factors, such as periods with low temperature, (lack of) humidity, or little day length, have an important role in the life cycle of many plants. Where a short-term environmental factor such as ambient light has an immediate effect on day-to-day morphogenesis (e.g. seedlings grown in the light are characterised by short hypocotyls and expanded green cotyledons and seedlings grown in darkness are etiolated, with elongated hypocotyls and closed cotyledons) long-term environmental factors often determine or have effect on the transition of a plant from one phase to another phase in its development, and in particular effect plants that are in a developmental pause in their development.
For example, periods with low temperatures occurring in the temperate climate zone during winters effect many transitional aspects of plants. Typical transitional events are stem elongation (bolting) and the onset of flowering and the formation of fruit (fruiting), another is the event that is seen as the break-through event of dormant seed, germination.
Some plant species need a developmental pause comprising a prolonged period of low temperature and/or little light or day length to induce flowering, and, in a process called vernalisation, for many plants traditionally artificial methods (such as forcing) are in widespread use to provide for the premature bolting, flowering and/or fruiting, or germination of a plant, seed, seedling or bulb to obtain commercially attractive plants or plant parts (seedlings, flowers or flowering plants, fruit) in a season in which otherwise such a product would nor or not natively be available.
In some species flowering and stem elongation may be induced at the same time. Other species need a period of low temperature to induce the elongation of the stem, a process that we will further refer to as bolting. In these species the process of flower induction and formation precedes the required period of low temperature, so the two processes are temporally separated. Yet other processes influenced by long-term environmental factors (e.g. low temperatures or dryness) are quiescence, dormancy and germination of seeds. These long-term environmental factors or processes often synchronise a plant's development to the seasons and/or climatic conditions of the place of growth, and reflect evolutionary adaptations to various conditions under which a particular plant or species grows.
Tulip is an example of a species used as a model to study these various aspects of long-term environmentally induced changes in plant growth, via the study of the process of bolting [4]. Another process where low temperature is often involved is seed dormancy. Low temperature induced stem elongation can be viewed as a form of dormancy breaking, and stratification, vernalisation and dormancy breaking in tree buds are similar processes driven by low temperature. In germination of dormant seed in Arabidopsis, besides the chilling requirement to overcome dormancy, other similarities between bolting of tulips and the germination of the dormant seed are the fact that all organs are formed within a protective structure to enable rapid growth after dormancy has been removed, reserve substances are stored in specialised leaf like organs (for example in the form of starch in the cotyledons of Arabidopsis, and also in the form of starch in the bulb scales of tulip), and during dormancy there is a low water potential [5].